1. Is your cabinet "just a cabinet" i.e. does it contain speakers only or are there other things inside? (I've known people who put lights in their cabinets) -- if the cabinet contains nothing but speakers, it can hardly be the source for hum.
2. Have you tried moving the cabinet somewhere else? It might be located at a spot with a hum source nearby (fluorescent lighting, switched power supplies such as TVs, computer screens,...)

I have seen too many strange problems caused by proximity to speaker magnets to rule out the speakers causing the hum. Because I prefer thick rubber feet to isolate the head from the cabinet vibrations anyway, I would suggest thicker rubber feet or thickening the bottom panel of the head.
Does the hum change if you turn the head to be at right angles to the cab.?

It's just a plain speaker cabinet. I have another home brewed head that works just fine with it. It's not the location either.

g1: the hum doesn't change when I rotate the head. I placed the chassis inside it's enclosure (which has thick rubber feet as you suggested) and I get the same hum. I have to lift the chassis about four inches off of the top of the cabinet before it goes away.

Try this, hold the head away from the cab, then hold you palm against the bottom of the head right under the input jack area. Does it hum that way? Or hold an AC cord from some operating piece of gear right under there. If either of these things cause hum, you may need shielding.

In commercial amps there is usually a coating of aluminum foil or screen or a metal plate to shield the open side of the chassis. Peavey even uses conductive paint for the purpose - can't rip that. But if someone made this amp, there may be a lack of shielding. That would allow exterior fields to couple into the amp.